Heretofore, a most useful HTSC in thin film superconductor technology has been YBa.sub.2 Cu.sub.3 O.sub.6+x (YBCO). However, there have been problems with the use of YBCO, including the fact that it undergoes a phase transition leading to twinning and stress. A more significant shortcoming with YBCO is the loss of oxygen when heated, which causes a difficult problem in the fabrication of multilayered devices. While it has been well-known that all HTSC cuprate materials lose oxygen upon heating, typically in the range around 450.degree. to 500.degree. C., up until now there has been no effective or practical superconductor material that retains oxygen when heated above about 500.degree. C. This oxygen loss is significant because superconducting properties, critical currently transition temperatures (T.sub.c) depend upon oxygen stoichiometry. The compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, where Me is a rare earth or alkaline metal ion and Nc is a 2+ ion selected from the group consisting of magnesium (Mg) and cadmium (Cd) overcome the shortcomings, drawbacks and limitations of other cuprate HTSC materials by being significantly more stable regarding oxygen loss, particularly in the temperature ranges at which thin film multilayered structures are fabricated. Typically for growth of multilayered structures, temperatures in the range about 550.degree. to 750.degree. C. are required for epitaxial growth of the dielectric and a temperature range of about 650.degree. to 800.degree. C. is required for the HTSC (YBCO).
The HTSC system La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x of the present invention is not only attractive for numerous device applications because of its superior oxygen stability as compared with YBCO, further such compounds do not undergo phase transition between the sintering and room temperatures. Additionally, thermal cycling in La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x compounds has demonstrated reversible oxygen losses of less than 1% when heated up to 1000.degree. C. in bulk samples, according to M. Nicolas et. al, which should permit easier preparation of multilayered structures and multilayered devices such as Josephson junctions, broadband impedance transformers and both flux flow and field effect transistors. The inventors herein have prepared thin films with compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, where Me is a rare earth or alkaline metal ion and Nc is a 2+ ion selected from the group consisting of magnesium (Mg) and cadmium (Cd), with substitutions for lanthanum (La) and calcium (Ca) and have been able to increase T.sub.c. The compounds of the present invention solve the oxygen loss problem of cuprate HTSC materials without suffering from any of the disadvantages, drawbacks and limitations of prior art compounds.
A superconducting bulk compound in the system La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x where z=0 and v=0, namely La.sub.3 Ba.sub.3 CaCu.sub.7 O.sub.16+x, known as La3317, was first prepared by Engelsbereg and the material was found to be a superconductor with a T.sub.c =80.2 K and to have a triple perovskite structure. The compound was tetragonal with c=3a (c=11.61, a=3.87). An earlier study by H. Fujishita et. al performed on a composition close to La3317 deduced that most of the calcium atoms occupy the central position of the unit cell occupied by the yttrium atoms in YBa.sub.2 Cu.sub.3 O.sub.6+x, (YBCO). Recently, Kao et. al reported that about half of the calcium atoms enter the yttrium sites and that the remainder are on barium (Ba) sites. Similar results have been achieved in studies of other La--Ba--Ca--Cu oxides by Fujishita and Wang, where much of the lanthanum is found in the barium sites. Engelsberg already studied the substitution of magnesium (Mg) and scandium (Sc) for calcium and he found a slight increase in lattice parameter when magnesium is substituted, c=11.71 .ANG. and a=3.91 .ANG., and he observed superconductivity below 30 K. Additionally, scandium substitution led to a multiphase material with no observed superconductivity. Wu et. al also investigated rare earth substitution for lanthanum in bulk samples. Based on these superconductor studies, the prior art only indicates a search for higher T.sub.c, materials, while still not satisfactorily resolving the long-felt shortcomings and disadvantages of cuprate HTSC materials' oxygen loss and phase transition problems. The compounds of the present invention now resolve those problems by being more stable with respect to oxygen such that these compounds can provide more effective, practical, cheaper and longer lasting superconductors.
Prior art describing similar compounds is found at A. W. Sleight and R. Ward, Inorganic Chemistry, 3 rd edition, p. 292 (1964);
S. Engelsberg, 176 Physica C 451, 1991; PA1 H. Fujishita et. al, 28 Japanese Journal of Applied Physics 754 (1989); PA1 H. C. Kao et. al, 9 Superconductor Science Technology 10 (1996); PA1 M. Nicolas et. al, 249 Physica C 377 (1995); PA1 C. M. Wang et. al, 39 Journal of Chinese Chemical Society 67 (1992); and PA1 D. S. Wu et. al, 212 Physica C 32 (1996).